1. Field of the Invention
The present invention relates to a thin film capacitor used as a stacked capacitor in a memory cell of a dynamic random access memory (DRAM) device.
2. Description of the Related Art
Generally, in a DRAM cell, a stacked capacitor is formed by a lower electrode layer, an upper electrode layer and an insulating layer. Particularly, in a 1 Gbit DRAM device, a perovskite-type oxide layer is adopted as the insulating layer therebetween to improve the dielectric constant, the leakage current characteristics (the insulating characteristics) and the chemical stability characteristics. The perovskite-type oxide layer is made of oxide represented by ABO.sub.3 where A includes at least one of metals belonging to II-group or Pb and B includes at least one of metals belonging to IV-group. For example, ABO.sub.3 is (Ba,Sr)TiO.sub.3 (BST) and (Pb,Zr)TiO.sub.3.
The perovskite-type oxide layer is formed by using a sputtering process, a chemical vapor deposition (CVD) process or a sol-gel process. In view of the step coverage characteristics, the CVD process is preferably used.
A first prior art thin film capacitor includes a perovskite-type oxide layer having a columnar structure (see: Eiji Fujii et al., "Preparation of PbTiO.sub.3 thin films by plasma-enhanced metalorganic chemical vapor deposition", Appl. Phys. Lett. 6513, pp. 365-367, Jul. 18, 1994). This will be explained later in detail.
In the first prior art thin film capacitor, however, the leakage current is increased. As a result, the insulating characteristics of the thin film capacitor are deteriorated, and also the breakdown voltage characteristics of the thin film capacitor are deteriorated.
A second prior art thin film capacitor includes a perovskite-type oxide layer having a granular structure (see: Takaaki Kawahara et al., "Influence of Buffer Layers and Barrier Metals on Properties of (Ba, Sr) Ti O.sub.3 Films Prepared by Liquid Source Chemical Vapor Deposition", Jpn. J. Appl. Phys. Vol. 36 (1997), pp. 5874-5878, September 1997, and Takaaki Kawahara et al., "Surface Morphologies and Electrical Properties of (Ba, Sr)Ti O.sub.3 Films Prepared by Two-Step Deposition of Liquid Source Chemical Vapor Deposition", Jpn. J. Appl. Phys. Vol. 34 (1995), pp. 5077-5082, September (1995). This also will be explained later in detail.
In the second prior art thin film capacitor, however, the dielectric constant is reduced.